drawing from life is a good way to learn anatomy. going by flat pictures can only take you so far, but it can be hard to find a willing model.
when drawing from imagination, it helps me to imagine my "subject" as a collection of very simple geometric shapes. spheres, cylinders, etc. and i build off of there. i imagine that goes for life drawing too, but i don't do very much of that.
i couldn't tell you what i do when i feel "inspired". i look at something, i combine it in some way or other with another thing i saw, and the rest is incomprehensible.
one of my monster designs was a combination of a character i saw with a hypodermic needle for a head, plus an earlier concept i made of people with compound eyes (fibers), plus another monster from a videogame. amalgamations like that.
really, one of the secrets to creativity is hiding your sources.
try going to the local library and draw people there candidly. i've done that only once, but it was pretty funny.
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וזה הכל אהובי, זה הכל.